We propose a model of mitochondrial DNA organization of Zea mays of multiple circular chromosomes of variable ploidy. Variant mitochondrial DNA organization affects both components of fitness: survivability of the organism and fecundity. Changes in mitochondrial DNA organization can result in male sterility and susceptibility to fungal toxins. The variant forms of mitochondrial DNA may be generated by intra- or interchromosomal rearrangements. We will study mitochondrial DNA organization by constructing a restriction endonuclease cleavage map of each chromosome. This will be facilitated by the production of individual R1 fragments of mitochondrial DNA in a lambda Charon 4A library we have constructed of normal mitochondrial DNA from two corn lines. The mapping procedure requires hybridizing radioactively labelled cloned probes to restriction digests of mitochondrial DNA prepared with a variety of restriction enzymes. We are also studying mitochondrial DNA organization by electron microscope to determine the distribution of different circular size classes. We have determined that the relative ploidy of individual size class chromosomes is determined by the nuclear genome of the organism. We plan to compare the physical organization of mitochondrial chromosomes of normal and variant lines by comparison of the distribution of chromosome size classes and by comparison of the cleavage maps of normal and variant mitochondrial DNA. This analysis should allow us to map the changes that confer male sterility and determine whether a specific sequence or region of one chromosome is involved or whether many different changes confer this important phenotype.